The present invention relates generally to an electron optics scheme for generation of high frequency electromagnetic energy and, more particularly, to a method and apparatus for extracting an electron beam from a cathode while preserving beam quality.
X-ray generating systems typically include an electron generating cathode and an anode in a sealed housing. The cathode provides an electron stream or current that is directed toward the anode. This focused electron beam is accelerated across the anode-to-cathode vacuum gap and produces x-rays upon impact with the anode. Because of the high power density generated at the location where the electron beam strikes the target, it is desirable to rotate the anode assembly. Many x-ray tubes therefore include a rotating anode structure for distributing the heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle. The axle supports a disc-shaped anode target as well as an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. The whole cathode and anode assembly is enclosed in a high vacuum environment.
One particular use of such x-ray generators is in the field of diagnostic imaging. Typically, in computed tomography (CT) imaging systems, for example, an x-ray source is collimated to emit a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray tube or generator and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray detectors typically include a post-patient collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom. Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
In order to generate an x-ray beam of sufficient strength for CT and other x-ray based diagnostic imaging modalities, cathode assemblies of x-ray tubes often provide close to 1 ampere of electron current. The electrons emitted from a cathode are accelerated across the vacuum gap of the x-ray tube to the anode by voltages on the order of 20 to 150 kVp. To achieve electron emission from a thermionic emitter, for example, a control voltage of about 10 V is applied across the tungsten filament, producing high temperatures and a current of about 7 amps in the filament. Therefore, adjustments to the cathode control voltage and/or current regulate the tube current.
The high voltage vacuum environment within many x-ray tubes presents additional considerations for cathode design. Some attempts to reduce the power demands of an x-ray tube cathode have utilized specially designed materials having lower work functions than ordinary thermionic filaments. Others have sought to incorporate field emitter (FE) arrays into cathode assemblies; however, in order to implement such a FE array into a cathode assembly, several issues have to be addressed. First, in order to extract the electron beam from the FE cathode, a certain electric field must be applied on the cathode. To minimize the voltage necessary for extraction of the electron beam from the cathode, a mesh grid is often used to enhance the field strength at the surface of the field emitter. Another consideration in the design of the FE array is the efficiency with which focusing of the electron beam is carried out so as to form a usable focal spot on a target. Certain beam optics must be designed to focus the electron beam into a desirable spot size. While traditional mesh grids provide efficient low voltage extraction of the electron beam from the FE cathode, the grids also can cause degradation in the beam quality and negatively impact formation of a usable focal spot. That is, the increased beam emittance of the electron beam after the beam hits the mesh grid prevents the beam from being focused to a small spot on the target. Thus, it is difficult to design a FE cathode having a highly compressed electron beam when utilizing such a mesh grid.
Therefore, it would be desirable to have an apparatus and method for minimizing the voltage necessary for extraction of the electron beam from the cathode, while still allowing for sufficient focusing of the electron beam so as to form a usable focal spot on a target. In particular, it would be desirable to have a mesh grid that allows for efficient low voltage extraction and beam focusing.